hi
Not seen the dvd (yet) but have always thought that it either was, or was supposed to represent a (living) tree. The way it just shines when you go in, and is so soothing to the touch, the fact that it doesn't have that "damp" feel that so may stones in tombs have. I've always been glad that when the Museum of Wales took the carved stone, that they left the tree stone.
Of course those are just my thoughts, but when they first excavated/broke in; "he crept forward on his hands and knees along the dreary vault, when lo! In a chamber at the further end a figure in white seemed to forbid his approach. The poor man had scarcely power sufficient to crawl backwards out of this den of spirits as he imagined".
Didn't work though, he still went back and pulled it down, to find a pile of bone lying near it.
sam